1. Technical Field
The present invention relates generally to radio frequency (RF) signal circuitry, and more particularly, to transmit-receive RF front end integrated circuits with increased receive sensitivity.
2. Related Art
Wireless communications systems find application in numerous contexts involving information transfer over long and short distances alike, and there exists a wide range of modalities suited to meet the particular needs of each. These systems include cellular telephones and two-way radios for distant voice communications, as well as shorter-range data networks for computer systems, among many others. Generally, wireless communications involve a radio frequency (RF) carrier signal that is variously modulated to represent data, and the modulation, transmission, receipt, and demodulation of the signal conform to a set of standards for coordination of the same. For wireless data networks, such standards include Wireless LAN (IEEE 802.11x), Bluetooth (IEEE 802.15.1), and ZigBee (IEEE 802.15.4), which are understood to be time domain duplex systems where a bi-directional link is emulated on a time-divided single communications channel.
A fundamental component of any wireless communications system is the transceiver, that is, the combined transmitter and receiver circuitry. The transceiver, with its digital baseband subsystem, encodes the digital data to a baseband signal and modulates the baseband signal with an RF carrier signal. The modulation utilized for WLAN, Bluetooth and ZigBee include orthogonal frequency division multiplexing (OFDM), quadrature phase shift keying (QPSK), and quadrature amplitude modulation (16QAM, 64QAM). Upon receipt, the transceiver down-converts the RF signal, demodulates the baseband signal, and decodes the digital data represented by the baseband signal. An antenna connected to the transceiver converts the electrical signal to electromagnetic waves, and vice versa.
Conventional wireless communication transceivers typically do not generate sufficient power or have sufficient sensitivity necessary for reliable communications. Accordingly, circuitry between the transceivers and the antenna referred to as the front-end module are utilized. The front-end module includes a power amplifier for increased transmission power, and/or a low noise amplifier for increased reception sensitivity. In most cases, wireless communication devices utilized in time domain duplex (TDD) systems utilized a single antenna, so the front end circuit had switch circuitry to rapidly toggle between transmit and receive functions, which included activating and deactivating the power amplifier and the low noise amplifier. Such control was possible via the enable line output from the transceiver, which may also have varying voltages to control gain or setting the bias current of the transistors in the amplifier circuitry.
Typical RF switch circuits were single pole, double throw switches, though subsequent RF front end circuits such as those disclosed by the present inventor in U.S. Pat. App. Pub. No. 2010/0203844 entitled “Radio Frequency Transceiver Front End Circuit” eliminated the RF switch. In general, such front end circuits utilize a matching network with a first port connected to the power amplifier, a second port connected to the low noise amplifier, and a third port connected to the antenna. With this matching network instead of the RF switch, it became possible to fabricate the front end circuit on a single semiconductor substrate.
In some embodiments, the matching network was configured such that a resistive component of an input impedance of the activated low noise amplifier was substantially equivalent to a resistive component of an output impedance required for the activated power amplifier. With communication systems that require substantially higher transmit power in relation to received signals such as WiMAX (IEEE 802.16), however, the receive chain of the front end circuit tended to exhibit increase noise figures. In particular, with higher transmit power levels, it is necessary for the output impedance at the power amplifier to be lower (i.e. below 5 Ohms). This is understood to result in a higher transformation coefficient from the antenna to the low noise amplifier input, and corresponding resistive losses that are associated with the matching network. Accordingly, there is a need in the art for transmit-receive RF front end integrated circuits with increased receive sensitivity.